Abstract
Asteroseismology of non-radial pulsations in Hot B Subdwarfs (sdB stars) offers a unique view into the interior of core-helium-burning stars. Ground-based and space-borne high precision light curves allow for the analysis of pressure and gravity mode pulsations to probe the structure of sdB stars deep into the convective core. As such asteroseismological analysis provides an excellent opportunity to test our understanding of stellar evolution. In light of the newest constraints from asteroseismology of sdB and red clump stars, standard approaches of convective mixing in 1D stellar evolution models are called into question. The problem lies in the current treatment of overshooting and the entrainment at the convective boundary. Unfortunately no consistent algorithm of convective mixing exists to solve the problem, introducing uncertainties to the estimates of stellar ages. Three dimensional simulations of stellar convection show the natural development of an overshooting region and a boundary layer. In search for a consistent prescription of convection in one dimensional stellar evolution models, guidance from three dimensional simulations and asteroseismological results is indispensable.
Highlights
Asteroseismology of non-radial pulsations in Hot B Subdwarfs offers a unique view into the interior of core-helium-burning stars
Some Subdwarf B (sdB) stars appear to be single and others occur in wide binaries, surveys have concluded that the majority are in close binaries with white dwarf or lowmass main sequence companions [3,4,5]
It has become clear that the physics of radiative levitation and gravitational settling are needed to reproduce the pulsational instabilities through iron element opacity bumps in their envelopes
Summary
Subdwarf B (sdB) stars are a class of hot (Teff = 20, 000−40, 000 K) and compact (log g = 5.0−6.2) stars with very thin hydrogen envelopes (MH < 0.01 M ) [1, 2] They form the so-called extreme horizontal branch (EHB) in the Hertzsprung-Russell diagram, where most of them quietly burn helium in their cores for ∼108 years. The balance between gravitational settling and radiative levitation creates a region with an overabundance of these iron group elements (especially Fe and Ni, [26]) in the envelopes of these stars, leading to an opacity bump The inclusion of both diffusion processes is important to explain the low atmospheric helium abundances, and to create the driving region for the pulsations. Asteroseismological analysis of sdB g-mode pulsators provides an exclusive window into the interior structure of core helium-burning stars. One has to take great caution in interpreting the details of the He-flash calculated by one dimensional stellar models
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